A sheet of expanded metal is conventionally made from sheet metal that is slit according to an arrangement of staggered parallel dashed lines, and stretched (expanded) perpendicular to the dashed lines into a structure of cross laths with interstices. Its manufacture is generally cheap. Moreover the sheets can easily be cut to a desired shape for a particular application, e.g. circular, for example by laser cutting.
The sheet of expanded metal forms a regular grid of openings, wherein each opening is defined by two substantially V-shaped pairs of strands, wherein the strands of each pair are adjoined in the direction of stretching and define a stretching angle, and two bonds interconnecting the two pairs of strands at the ends of the V-shapes. The openings of expanded metal have substantially rhombical or frustro-rhombical shape. Substantially rhombical shape (the shape of an equilateral parallelogram) is obtained when the slits along a virtual dashed line are much longer than the non-slit parts (so-called bonds) between them. After stretching of the slit sheet metal, the bonds form corner points each connecting four so-called strands of equal length. If the non-slit parts between the slits are relatively long, than a substantially frustro-rhombical shape is obtained. By frustro-rhombical it is meant that two opposite corners of a rhombical shape are cut off along a pair of parallel lines.
The strands after stretching of expanded metal are not perfectly straight but often slightly S-shaped; i.e. curved at the connections with the bonds, with a substantially linear central part in between. It will be understood that the V-shape of pairs of adjacent (neighboured) strands in the direction of stretching is somewhat distorted. The stretching angle is suitably defined by the central parts of the strands forming the (distorted) V-shape.
Known expanded metal is stretched to a maximum of 90 degrees, resulting in approximately square openings, as can for example be obtained from Sorst Streckmetall GmbH, Hannover, Germany.
Also, when expanded metal is stretched out, the strands are twisted out of the plane of the sheet metal, unless the sheet is subsequently flattened again. The slight S-shape of the strands and their attachment to the bonds has the consequence that the openings have no more than two mirror symmetry axes, which can be along and perpendicular to the direction of the stretching. This will be discussed in more detail with reference to the drawings.
To account for deviations of the openings in expanded metal from a perfect rhombical or frustro-rhombical shape are, the term ‘substantially rhombical or frustro-rhombical shape’ is used hereafter. The term therefore encompasses perfect and imperfect or distorted such shapes.
International Patent Application No. PCT/EP03/01074 in the name of Applicant, which was unpublished at the priority date of the present application, relates to the use of expanded metal as support for a bundle of tubes, such as a support baffle in a heat exchanger, in particular a shell-and-tube heat exchanger.
A shell-and-tube heat exchanger normally comprises a cylindrical vessel in which a bundle of parallel tubes are arranged extending in longitudinal direction of the vessel. It is an indirect heat exchanger in which heat is transferred between a fluid passing through the tubes of the tube bundle (the tube side) and a fluid passing through the space outside the tubes (the shell side). Details of the shell-and-tube heat exchangers can for example be found in Perry's Chemical Engineers' Handbook, 6th edition, 1984, McGraw-Hill Inc., page 11-3 to 11-21. The ends of the tubes of the tube bundle are secured to a tube sheet. The heat exchanger can include two tube sheets, one at each end of the cylindrical vessel, or a single tube sheet at one end of the cylindrical vessel in the event the heat exchanger is a U-tube exchanger.
The intermediate portions of the tubes have to be supported as well, for example to prevent damage to the tubes due to vibrations caused by the fluid flow. To support the intermediate portions of the tubes, a support can be used including transverse support plates that are spaced apart along the length of the tubes.
A conventional support comprises segmental baffles, and there are several kinds discussed in Perry's. Baffles do not only support the tubes, but also influence the fluid flow through the shell side. Therefore the design of a baffle is as well determined by heat-transfer considerations.
U.S. Pat. No. 4,143,709 discloses a support for a bundle of parallel tubes, which support comprises a plurality of transverse support plates spaced apart along the length direction of the tubes to be supported. Each plate is integrally formed from a single sheet wherein a plurality of holes is cut on a regular grid, each of which holes is large enough to accept a plurality of tubes. The holes can be generally rectangular-shaped, generally square-shaped, generally triangular-shaped or generally rhombical (diamond-shaped).
At least one of the support plates of the known support is disposed out of phase or staggered. Tubes extending through the same hole in one support plate extend through different holes in another support plate, so that the cooperating adjacent plates support the tubes from opposite sides against lateral movement.
However, applicant has found that the deviations from perfect rhombical or frustro-rhombical shape make it difficult to use expanded metal in a staggered arrangement like in U.S. Pat. No. 4,143,709, since at least two different types of passageways are formed with slightly different cross-section and shape, which is undesirable for optimum tube support.
It is an object of the invention to provide a new sheet of expanded metal, in particular expanded metal with optimum properties for use as a tube support.
It is a further object to provide a new method for manufacturing expanded metal.